The use of a binder is known in an electrode of a battery. Representative examples of the battery having the electrode comprising the binder include a lithium-ion secondary battery.
Since the lithium-ion secondary battery has a high energy density and a high voltage, the lithium-ion secondary battery is used for electronic apparatuses, such as a mobile phone, a laptop computer and a camcorder. Recently, because of a rise of consciousness to environmental protection, and enactment of related laws, the application in vehicles, such as an electric vehicle and a hybrid electric vehicle, and the application of a storage battery in electric power storage for home are also progressing.
The lithium-ion secondary battery generally comprises a negative electrode, a positive electrode, a separator, a liquid electrolyte, and an electrical collector. With respect to the electrode, the negative electrode is obtained by applying a coating liquid which comprises a negative electrode active material, such as graphite and hard carbon capable of intercalating and deintercalating a lithium ion, an electrically conductive additive, a binder, and a solvent onto an electrical collector represented by a copper foil, and drying the coating liquid. Recently, a dispersion comprising a styrene-butadiene rubber (abbreviated as “SBR” hereafter) dispersed in water is generally used as the binder.
On the other hand, the positive electrode is produced by mixing a positive electrode active material such as lithium cobaltate and spinel-type lithium manganate, an electrically conductive additive such as carbon black, and a binder such as polyvinylidene fluoride and polytetrafluoroethylene, dispersing them in a polar solvent such as N-methylpyrrolidone to prepare a coating liquid, then coating the coating liquid, as in the negative electrode, on an electrical collector foil represented by aluminum foil, and drying the coating liquid.
These binders for lithium-ion batteries need to increase the addition amount of the binder, in order to secure the bondability, thereby causing the deterioration of the performances which is a problem of these binders. Since N-methylpyrrolidone is used for the slurry solvent, an aqueous binder is required from a viewpoint of a recovery, a cost, toxicity and an environmental load. However, the use of a SBR-base binder, which is an aqueous binder, has the problem that oxidization degradation is caused under a positive electrode environment. Therefore, the binders comprising polyvinylidene fluoride and/or polytetrafluoroethylene dispersed in N-methylpyrrolidone as a dispersing solvent are still used as the binder of the positive electrode. Urgently required is the development of the binder which is excellent in the bondability between an electrical collector and an active material, and between active materials, has a low environmental load, is an aqueous binder, and is suitable for manufacture of the electrode for secondary batteries having high oxidation resistance.
In order to solve the above-mentioned problems, Patent Documents 1 and 2 propose a binder comprising a copolymer which comprises an aromatic vinyl, a conjugated diene, an ethylenically unsaturated carboxylic acid ester and an unsaturated carboxylic acid (Patent Document 1), and a binder comprising an aqueous polymer dispersion selected from a styrene-butadiene polymer latex and an acrylic emulsion (Patent Document 2).
Further, Patent Documents 3 and 4 propose a binder comprising a copolymer which comprises an aromatic vinyl, a conjugated diene, a (meth)acrylate ester and an ethylenically unsaturated carboxylic acid (Patent Document 3), and a binder comprising a polymer which comprises a difunctional (meth)acrylate (Patent Document 4).
However, if these binders are used for an electrode (a positive electrode and/or negative electrode), a charge/discharge cycle is deteriorated under the conditions of high temperature. Particularly, if these binders are used for the positive electrode, there is the concern that a problem of oxidation resistance under conditions of high voltage is caused to deteriorate the battery properties.